Figure REF  shows the global dependence of the [C ii]/FIR ratio on the dust optical depth \(\tau _{160}\) . As opposed to points lying in OMC1, studied in [1]}, we do not observe a linear reciprocal trend in the [C ii]/FIR ratio with \(\tau _{160}\)  on larger scales. [1]} expect \(\log _{10} I_{\mathrm {[C\,\textsc {ii}]}}/I_{\mathrm {FIR}} \simeq -\log _{10}\tau _{160}+C\)  for a cloud slab of constant dust temperature viewed in face-on geometry. The fact that we do not observe this behavior globally indicates that, unlike for OMC1, the temperature varies on large scales. However, there seems to be a decreasing trend of [C ii]/FIR with increasing \(\tau _{160}\) . Low [C ii]/FIR ratios are found toward the OMC4/ISF region, where \(\tau _{160}\)  is large due to a large column of cooler dust. This cool dust likely emanates from deeper layers in the cloud that are not exposed to FUV radiation.
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